This invention relates to a lens mount, or more generally to a device for supporting an optical element such as a refracting lens. In particular, this invention relates to such a kinematic lens mount requiring reduced clamping force, say, for a projection optical system of an exposure apparatus used in a lithography system for fabricating micro-devices or forming masks such as reticles.
It has been known to support an optical element such as a deformable mirror, and more particularly a thin-membrane mirror, by means of many high-stiffness actuators such as PZT actuators, as described, for example, in U.S. Pat. No. 5,037,184 issued Aug. 6, 1991 to Ealey. These many actuators overconstrain the mirror, and overconstrained mirrors have disadvantages for precision control.
Deformable mirrors with low-stiffness force-type actuators for controlling deformation without overconstraint were disclosed by John Hardy (“Active Optics: A New Technology for the Control of Light,” IEEE, Vol. 60, No. 6 (1978)) but high-stiffness kinematic mounts are used for controlling the position in six degrees of freedom. Kinematically constrained deformable mirrors with force actuators require some other means for controlling or adjusting the rigid body position.
A quasi-kinematic lens mounting assembly was disclosed in U.S. Pat. No. 6,239,924 issued May 29, 2001 to Watson, adapted to support a lens on a set of mounting seats and also provided with a set of soft mounts for further distributing the gravitational load without overconstraining the lens. A device for kinematically holding an optical element was also disclosed by Shibazaki (WO Publication 02/16993 A1 published Feb. 28, 2002), adapted to support an optical element on bearing surfaces (like seats) on a base member and provided with clamping members each including a pad member with a flexible thin plate portion through which the optical element is clamped by means of bolts.
The optical member to be supported may be very fragile, such as those comprising CaF2. An excessively large clamping force thereon may damage it or cause intolerable deformations. Thus, it is desirable to reduce the clamping force as much as possible while maintaining friction force needed for supporting such an optical element.